1. Field of the Invention
The present invention relates to a foreign exchange station (FXS), and in particular, to a ring relay control unit for use in the FXS that is capable of protecting a ring relay and reducing noise.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram illustrating the construction of a conventional ring relay control unit of an FXS. In FIG. 1, the ring relay control unit 100 of a conventional FXS comprises a main control unit 130 for controlling a relay interrupt command, a ring signal generator 110 for generating a ring signal which is transferred to a subscriber's phone 150, a subscriber line interface circuit (SLIC) for interfacing a call signal to the subscriber's phone 150, and a relay 140 for selectively transferring the call signal and the ring signal to the subscriber's phone 150 from the SLIC 120 and the ring signal generator 110 in accordance with a relay interrupt command by the main control unit 130.
The ring relay control unit 100 constructed as above interfaces the relay 140 to a terminal 2 when the subscriber's phone 150 is engaged in communication, and interfaces the relay 140 to a terminal 1 when transferring the ring signal to the subscriber's phone 150. Thus, the ring relay control unit 100 selectively transfers the ring signal and the call signal to the subscriber's phone 150.
FIG. 2 is a diagram illustrating the timing of generating a ring relay control signal by the conventional ring relay control unit 100. In FIG. 2, a ring signal generated by the ring signal generator 110 continuously generates sine waves. The ring relay control unit 100 interfaces the relay 140 to the terminal 1 in accordance with a relay interrupt command sent by the main control unit 130, and transfers the ring signal generated by the ring signal generator 110 to the subscriber's phone 150.
Thus, the voltage applied to the relay 140 from the ring signal generator 110 is variable, dependent on the timing of the generation of the relay interrupt command transferred from the main control unit 130. This means that the voltage applied to the relay 140 from the ring signal generator 110 is variable in a wide range depending on the point of time when the relay interrupt command is generated as shown in FIG. 2.
Such variable voltage loaded on the relay 140 poses a danger of damaging the relay 140 and has the drawback of generating switching noise due to an interrupt of an overvoltage signal.
Also, in the case where the relay 140 interfaced to the terminal 2 transfers the call signal to the subscriber's phone 150 from the SLIC 120, the ring signal generator 110 continuously generates the ring signals, thereby deteriorating the efficiency of and wasting the electric power.